The present invention relates to the storage of large quantities of liquefied gas, most advantageously natural gas, in a rigid or semirigid storage vessel which, in a typical embodiment, is located underground, but may also conveniently be located above ground.
Three different concepts have been utilized in existing cryogenic liquefied gas storage systems. These concepts have respectively involved the use of above-ground tanks, below-ground tanks, and in-ground storage. In-ground tanks, that is, those which store liquefied natural gas directly in a frozen hole in the ground, typically use the earth itself as a liquid container. In a few instances, a flexible, moisture-impervious liner has been incorporated. Although the heat leak into these tanks is usually excessive, a few of them have enjoyed limited success. A principal problem with this approach is that the success or failure of the project depends almost entirely on the effect the cryogenic temperatures have on the soil or rock formation in which the container is constructed. In many cases, subjecting the soil or rock to cryogenic temperatures results in fracture and fissure formations which increase the heat leak to the point where the tank is not usable and must be abandoned.
For this reason, most tanks are constructed either above ground, as by forming a concrete or metallic outer tank structure with a metal inner tank structure separated from the outer tank by a liner of insulation; or underground, as by fabricating tanks submerged wholly or partially in the ground. In above-ground vessels of this prior art type, the outer tank may or may not have an independent floor member, as dictated by construction practicalities. In such installations, a void may be left between the tank and the surface of the ground, which void may be filled with insulation. Alternatively, a second, inner tank may be spaced from the outer tank, and the void therebetween may be filled with insulation. In each case where an inner tank, used for storing liquefied gas, is separated from an outer container, either a second tank or the surface of an excavation, supporting structure such as peirs have been used to support the inner tank above the level of the excavation floor or the floor of the outer tank. These piers thus elevate the inner tank to produce a void which may be filled with insulation material. While the use of such insulation material substantially improves the quality of the storage tank by limiting heat leaking into the floor of the liquefied gas storage vessel, the use of piers or other rigid supporting structures presents a substantial heat leak problem. Thus, heat flows directly from the ground below the excavation and through the support structure in underground tanks. Similarly, heat flows through the supporting structure into the liquefied gas from the underlying ground surface in above-ground tanks. An installation which avoids this inherent difficulty is described in U.S. Pat. No. 3,701,262 entitled "MEANS FOR THE UNDERGROUND STORAGE OF LIQUEFIED GAS," issued to Joseph A. Connell and Anthony J. Baranyi and assigned to the assignee of the present invention. In the storage system described in that patent, an excavation in the ground is lined with a flexible, impervious liner, and insulation such as granular perlite is placed in bags which are stacked on the floor and against the walls of this lined excavation. A second flexible, impervious liner is then placed against the bagged insulation and is used to directly store the liquefied gas. Since this inner liner is flexible and conforms to the bagged perlite, the perlite is not permitted to creep or migrate from beneath the storage vessel. In some installations, however, as for example where excessive earth movement would place excessive stress upon a flexible inner liner, a rigid or semirigid interior tank is required.
In installations such as that shown in patent 3,701,262, it is desired to have sufficient insulation between an inner and outer tank or wall to permit control of the thickness of a frozen wall of earth surrounding the tank. The use of piers or columns for supporting a semirigid inner tank would interfere with this control, since it would cause substantial freezing of the earth beneath the tank which could not be adequately controlled.